Bicycling is a popular activity having world wide appeal. Bicycles are used in a variety of settings including transportation, racing, (for example, road racing and mountain bike racing), and work environments (for example, delivery services and bicycle police patrols). The majority of bicycles, however, are used for recreational purposes. A 1991 study by NFO Research and reported in American Demographics Magazine, 1995, estimated that there were 33 million recreational bicycle riders in the United States. The survey classified those riders into four categories: enthusiasts, moving ups, casuals, and infrequents. The casuals and infrequents categories account for 90% of the riding population and 70% of bicycle industry revenues.
With wide use has come improved bicycle technology. Historically recent advances in bicycle technology have produced lighter and stronger bicycle frames, improved breaking systems, and improved propulsion systems. Propulsion system advances include the multi-speed gear system. While there are many variations of multi-gear systems for bicycles, the most common system includes, one, two, or three sprockets located adjacent to the pedaling crankshaft operationally connected via a chain to a multi-sprocket system located adjacent to the rear wheel hub. The rear wheel gear system also includes a derailleur. The derailleur connects via a shifting cable to a lever located on either the bicycle frame or bicycle handlebar. To change gears, i.e. adjust the chain from one sprocket to another sprocket, a rider manipulates the lever which causes the derailleur to move. Movement of the derailleur causes the chain to move from sprocket to sprocket. Each of the sprockets found on the rear wheel hub is of a different size. Thus, as the chain is selectively moved between the sprockets, the gear ratios are altered resulting in a change of gears. For instance, if the chain is moved to a larger sprocket, pedaling becomes easier, if moved to a smaller sprocket, pedaling becomes more difficult. An example of such a system is seen in U.S. Pat. No. 4,185,510 to Guy.
Multi-speed bicycles are very popular inasmuch as they make bicycles more versatile. However, gear shifting makes a bicycle more complicated, and can also lead to injury as a result of a rider diverting his attention from the roadway to the shifting mechanism during a bike shift. Support for these statements is found in the 1991 NFO Research study which noted that casual and infrequent riders avoid visiting bicycle shops as a result of the fear of being overwhelmed by technical talk, and that riders were looking for an uncomplicated way to ride a bicycle.
Even before the 1991 survey, advancements had been made in multi-gear bicycle systems. Among these advances are automatic gear shifting mechanisms. Automatic gear shifting mechanisms are designed to make bicycling more enjoyable and less dangerous by providing a means to take advantage of the multi-speed capabilities of a bicycle while, at the same time, reduce the complexity associated with the multi speed systems. Automatic gear shifting mechanisms rely upon either torque or centrifugal force. Representative prior art example of an automatic gear shift system relying on torque is seen in U.S. Pat. No. 3,769,848 to McGuire which teaches an automatic gear shifting mechanism for multi-speed bicycles consisting of a spring activated gear shift mechanism mounted near the rear wheel hub sprockets. Shifting occurs in response to increased torque applied to the wheel hub. Torque is created by the rider applying increased or decreased pressure to the pedals. As torque increases, the device shifts to a higher gear, and as torque decreases, it shifts to a lower gear. A shortcoming to torque reliant devices is that they require a rider to alter the amount of pressure applied to the pedals, the torque being transferred to the rear wheel in order to shift the rear wheels. Increasing the torque too drastically may cause the bike to shift more gears than is required, making bicycle pedaling more difficult than need be. Also, increased torque adds unneeded stress to the legs, particularly the knees and hips, which can lead to injury. When attempting to downshift, a rider may also experience multiple gear sprocket shifts if the amount of torque is decreased substantially. A further shortcoming of these devices is that when a rider "coasts" or does not pedal, the mechanism is placed in position for a downshift. This can cause downshifting followed almost immediately by upshifting when the rider resumes pedaling. Such movement is not only awkward but can also lead to injury.
A representative example of a centrifugal force type shifting mechanism is found in U.S. Pat. No. 4,352,504 to Cotter which teaches a speed sensitive automatic transmission for bicycles. The device includes a rotatable shaft connected to the rear wheel and having a plurality of outwardly extending arms fixed to the shaft. Each outwardly extending arm has a weight member slidably connected to it. As the speed of the tire either increases or decreases, the weights move along the plurality of outwardly extending arms. The movement of the weights causes an energy transfer via a cable to the rear wheel portion of the device which causes the gears to shift. A shortcoming of this device is that it is dependent upon the speed of the rear tire and not necessarily the speed the rider is pedaling. Thus, if a rider is traveling downhill and coasting, the tire is traveling at a high rate of speed while the rider is pedaling slowly or not at all. The device, however, will cause a shifting of the gears when it is either unnecessary to do so or will actually shift to an undesirable gear. For instance, if the tire is traveling at a high rate of speed, the device will shift to a higher sprocket. This may not be desirable in a situation when the rider will end a downhill run and immediately engage in an uphill run which requires a lower gear.
Another example of a centrifugal force type automatic bicycle transmission is seen in U.S. Pat. No. 4,571,219 to Bredan et al. which teaches an apparatus utilizing speed actuators which respond to the centrifugal force created by the rotating bicycle wheel. The speed actuators are attached to the sprockets of the rear wheel and, through a chain mechanism, attach to the gear shifting mechanism. This device also contains a master cylinder containing fluid and a piston. As the speed of the rear wheel increases or decreases, the actuators move along the chains, causing the fluid to move the piston, resulting in the shifting of gears. Similar to U.S. Pat. No. 4,352,503, this device suffers from the same shortcomings inasmuch as the gear shifting mechanism depends on the speed of the rear wheel as opposed to the speed with which a rider is pedaling.
Yet another example is seen in U.S. Pat. No. 4,743,014 to Vasin which teaches an automatically variable gear ratio drive. This device does not require rear wheel gear sprockets. Instead, a single chain or belt roller mechanism is attached to an axially mounted spring. As the speed of rear wheel bicycle increases, the spring compresses, causing the entire mechanism to move closer to the wheel hub. This, in turn, causes movement of a pulley mechanism mounted near the front crank shaft. This movement causes a tensioning of the chain, resulting in increased difficulty in pedal movement. Conversely, slower pedaling causing expansion of the spring resulting in less tension in the chain or belt and easier movement of the pedals. A shortcoming of this device is that it is not suitable for conventional multi-speed bicycles inasmuch as the rear wheel multi-sprocket system and derailleur are absent. A further shortcoming of this device is that it appears to require a specially modified frame which increases the cost of a bicycle.
Final examples of a centrifugal force type automatic transmission for multi-speed bicycles are seen in U.S. Pat. Nos. 5,163,881, 5,295,916, and 5,445,567, all to Chattin. These patents teach an automatic transmission for bicycles utilizing a centrifugal force mechanism for gear shifting. The mechanism consists of weights connected to a rotatable shaft positioned through a base member to the derailleur. A collar is pivotally connected to each weight in such a manner that, upon rotation of the shaft, movement of the weight results which, in turn, causes movement of the collar. The collar is operatively connected to linkage members of the derailleur, the linkage members causing the derailleur to move and a gear shift results. A shortcoming of this device is that the principle functional features of the device are exposed to the elements and are subject to damage or faulty shifting.
A further shortcoming of this device is that gear shifting is controlled by the centrifugal force created by the speed of the rear wheel and not the pedaling cadence of the rider. Thus, undesired or unnecessary gear changes can occur.
There is need, therefore, for an improved automatic gear shifting mechanism for multi-speed bicycles which shifts gears based upon the speed of the rider's pedaling cadence as opposed to the speed of the rear wheel or applied torque.